A communication system exists where calls can be established via a public communication network such as the Internet using a peer-to-peer communication system. Currently, a system exists under the trade mark Skype which uses voiceover internet protocol (VoIP) packets to establish voice calls between users of the system. Calls are established by client software executed at client terminals in the system. The client software establishes a call instance between user terminals. A call instance can be used to convey voice, video, IMS or other types of data. Suitable internet packet protocols are used to convey the data.
Users of the system have in common the fact that they are all registered with a common provider of this system and this provides at least a first level of familiarity between users. That is, to the extent that the provider of the system monitors the behaviour of its users, there is a certain desirability of users of the system to want to deal with other users of the system as opposed to non users, where they need to access a service.